The tiger shark (Galeocerdo cuvier) is a large (up to 5.5 m Total Length) apex predator found in tropical and warm-temperate warm waters worldwide. Tiger sharks occur in a wide variety of marine habitats including those associated with continental shelves, oceanic islands and atolls, and also range extensively into open-ocean. Tiger sharks are opportunistic predators which consume a diverse array of taxa including fishes, other sharks, shellfish, reptiles, mammals and birds. Their diet changes as they grow. In Hawaii, small (<2.3 m) tiger sharks feed primarily on reef fishes and octopus, whereas larger individuals consume a variety of larger prey including other sharks, turtles, mammals and crustaceans.

Tiger sharks are one of the shark species most frequently implicated in attacks on humans, and although shark attacks are extremely rare overall, the few that do occur are often widely publicized by the media resulting in heightened public fear and calls for management actions to increase public safety. Shark attacks in Hawaii during the latter half of the twentieth century resulted in shark culling programs aimed at reducing shark attack risk by removing large tiger sharks from coastal waters. However, these control programs were discontinued after scientific studies showed tiger sharks did not occupy small coastal territories, as had been assumed, but were instead extremely wide-ranging.

Like shark culling programs, ocean use guidelines aimed at reducing shark attack risk are also based on assumptions about shark movement patterns and feeding behavior. For example, one common recommendation for reducing the risk of shark attack is to “Stay out of the water at dawn, dusk and night when some species of sharks move inshore to feed”. However, in Hawaii most (67%) shark attacks occur during daytime between the hours of 10am and 4pm, coinciding with peak time for in-water recreational activities.

We need to understand tiger shark long-term movement patterns to determine whether these are related to the rare incidences of shark attacks on humans.

Research Questions

We asked four specific questions about tiger shark long-term movements in the Hawaiian Islands:

Do tiger sharks show long-term fidelity to coastal sites?

Is there significant spatial and temporal overlap in long-term habitat use among individual sharks?

Do tiger shark movement and habitat use patterns correspond with the spatial and temporal patterns of shark attacks on humans?

Methods

We captured 15 tiger sharks and surgically implanted them with small ultrasonic transmitters. We stationed underwater listening stations (acoustic receivers) around the islands of Hawaii, Maui, Kahoolawe and Oahu. These underwater receivers listen continually for the presence of sharks implanted with coded pulse acoustic transmitters (within a detection range of up to 1000m). Receivers are periodically retrieved by divers and downloaded to find out which sharks have visited, when they came and how long they stayed at each location. We used this system to remotely track individual tiger sharks for up to 892 days (2.4 years).

Tiger sharks were wide ranging, swam between islands and patrolled up to 109 km of contiguous coastline. Visits to specific acoustic receiver sites were typically brief (mean duration 3.2 min), unpredictable and interspersed by absences of weeks, months or years. This pattern may be an optimal foraging strategy for capturing risk-averse prey. Tiger sharks may have to move on soon after arriving in an area because the element of surprise is quickly lost and potential prey become wary and difficult to catch. Juvenile tiger sharks were significantly wider ranging and less frequently detected than mature females. Juveniles may be avoiding predation by larger individuals, or exploring to find suitable home ranges. Tiger sharks may also switch movement patterns and foraging strategies to take advantage of different prey types, restricting their movements to exploit seasonally abundant and naïve prey.

We continue to develop and expand our tiger shark research program, and are gradually uncovering previously unknown facets of their behavior. To date we have implanted over one hundred tiger sharks with coded acoustic tags, and have also equipped a number of these sharks with satellite transmitters (SPOT & PAT tags) to gain a more comprehensive understanding of their movements. We are monitoring the long-term movements of these sharks with a listening array of over 100 underwater receivers deployed throughout the 2,500 km Hawaiian Archipelago, from Kure Atoll to Hawaii Island. We have also been using tri-axial accelerometer dataloggers to reveal further details of tiger shark behavior and swimming mechanics.

Previous Tiger Shark Research

Our current and ongoing tiger shark research grew out of an initial study (1993-1998) where we used a boat equipped with tracking equipment to follow individual tiger sharks for up to 50h. This study provided us with the first insights into the wide-ranging behavior shown by tiger sharks in Hawaii. Click here for further information on the tiger shark active tracking study.